Hydraulic fracturing is a technique commonly used to break open subterranean shale and rock formations to enhance the ease of extracting materials, most often oil and natural gas, from such formations. In this technique, water is mixed with sand and chemicals, and the mixture (“fracking fluid,” or “fracturing fluid”) is injected at high pressure into a wellbore to create small fractures.
A hydraulic fracture is formed by pumping the fracturing fluid into the wellbore at a rate sufficient to increase pressure downhole at the target zone to crack the surrounding rock. Once cracked, the fracture fluid continues further into the rock, extending the fracture. In order to maintain “fracture width” open, or slow its decline, a material that includes a solid particle having sufficiently high crush strength, such as grains of sand, ceramic, or other particulates is introduced into the injected fluid. This material is referred to as a “proppant,” and it prevents the fractures from closing when the injection is stopped and the pressure of the fluid removed.
Once the cracks or fractures are opened in the well section, the fracking fluid rushes in to fill them. It is the proppant that actually holds these fractures open or at least impedes the process of the cracks closing up after the fluid pressure is removed.
Proppants are designed to hold open the cracks in a formation for the life of the well, which is often from 5 to 45 years. Types of proppants include silica sand, resin-coated sand, ceramics and bauxite, among others. The most commonly used proppant is silica sand, though proppants of uniform size and shape, such as a ceramic proppants, may also be employed. Proppant pack may be permeable to oil or gas under high pressures, the interstitial space between particles of proppants may be sufficiently large and have the mechanical strength to withstand closure stresses and hold fractures open. The proppant is spherical, or nearly spherical to maximize the voids between particles, thereby allowing for a maximum flow of oil and gas past the proppant into the main portion of the well. The measure of how easily fluids can pass through a formation or through a proppant is known as its “conductivity.”
Untreated sand, when used as a proppant, is prone to generation of significant fines (i.e. finely ground sand) and flow-back of the proppant, especially under higher formation stresses. Synthetic resin coatings can be employed to provide particle-to-particle bond in order to minimize or eliminate flowback and to reduce the fine formation. Different synthetic resins can be used in proppant coatings including phenol resins, epoxy resins, polyurethane resins, furane resins, polyurea resins, etc. With some coatings, the synthetic resin is not completely cured until the coated proppant is placed into the well. The coated, partially cured proppant is free-flowing but it is still capable of providing particle-to-particle bond at the temperature and pressure encountered downhole. There are a number of performance issues associated to the use of partially cured proppants including storage stability, premature bonding and the need of an activator for lower temperature wells.
It would be desirable to provide a proppant coated with a synthetic resin that is a free flowing product and presents high crush resistance, minimum flowback, good conductivity and a bond strength significantly higher than commercially available alternatives without the need of an activator, when used for hydraulic fracturing operations in low temperature wells.